1. Field of the Invention
The invention is generally related to nuclear fuel assemblies and particularly to the fabrication of spacer grids used in nuclear fuel assemblies.
2. General Background
Fuel assemblies for nuclear reactors are formed from a number of fuel rods held in position radially by spacer grids located along the length of the fuel rods. The spacer grids are formed from slotted metal strips containing stamped features, which are crisscrossed on edge to define individual cells for each fuel rod. The crossing metal strips are aligned at approximate right angles to each other and then welded together at the intersections of the strips. A clamping fixture is used to hold the strips in position after they are properly aligned. The clamping fixture is then held by a welding fixture that is attached to the welding system. The welding system includes a laser, a vacuum weld chamber, and a mechanism that positions, rotates, and inverts the weld fixture as necessary to accommodate the intersection welding on both sides of the spacer grid. Typically, the inner strips of the spacer grid are meshed with each other to form the egg crate, aligned, and the intersection welds are made on the inner strips. The welded inner strips are removed from the weld system and weld fixture and the outer grid strips are positioned onto the welded inner strips and held in place by deformation of the inner edge weld tabs. The grid, with the outer strips attached, is placed into the edge weld system that includes a weld chamber, an indexing fixture for positioning the grid, and weld heads. The edge welding system does not weld the corners of the grid. The corners are manually welded, which requires a high level of operator attention and intervention. As a result, weld quality is variable and the edge welding process requires as much as one hour per spacer grid. The present use of different mechanical setups leaves a need for a more efficient means of welding spacer grids.